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We report the discovery of Mothra, an extremely magnified monster star,likely a binary system of two supergiant stars, in one of the strongly lensedgalaxies behind the galaxy cluster MACS0416. The star is in a galaxy withspectroscopic redshift $z=2.091$ in a portion of the galaxy that is parsecsaway from the cluster caustic. The binary star is observed only on the side ofthe critical curve with negative parity but has been detectable for at leasteight years, implying the presence of a small lensing perturber.  Microlenses alone cannot explain the earlier observations of this object madewith the Hubble Space Telescope. A larger perturber with a mass of at least$10^4$\,\Msun\ offers a more satisfactory explanation. Based on the lack ofperturbation on other nearby sources in the same arc, the maximum mass of theperturber is $M< 2.5\times10^6$\,\Msun, making it the smallest substructureconstrained by lensing above redshift 0.3. The existence of this millilens isfully consistent with the expectations from the standard cold dark mattermodel. On the other hand, the existence of such small substructure in a clusterenvironment has implications for other dark matter models. In particular, warmdark matter models with particle masses below 8.7\,keV are excluded by ourobservations. Similarly, axion dark matter models are consistent with theobservations only if the axion mass is in the range $0.5\times10^{-22}\, {\rmeV} < m_a < 5\times10^{-22}\, {\rm eV}$.

JWST's PEARLS: Mothra, a new kaiju star at z=2.091 extremely magnified by MACS0416, and implications for dark matter models